it is known to open or unlock the door of a vehicle or perform another function by operation of a miniature remote unit or electronic key which is generally carried on the key chain of the vehicle operator. By pressing a button on the remote unit a coded radio signal is transmitted to a base unit on the vehicle and, if it recognizes the code as an authorized code, the base unit unlocks the door or performs another prearranged function. It is recognized that such systems are not limited to controlling vehicle functions but might as well control garage doors, security gates, or home entry, for example.
Since it is a common practice for an operator to use more than one vehicle it is desirable to provide the remote unit with the capability of separately controlling several different base units mounted in different vehicles or in non-vehicular stations. This would avoid the necessity of carrying or obtaining a separate remote unit for each base station. It is also desirable to avoid handling the electronic key and pushing the button to activate the unit since it is sometimes inconvenient to do so. To carry out a passive control which needs no button, it has been proposed to provide a system in which the mere presence of the electronic key in the immediate vicinity of the base station is sufficient to activate the base station function. In conjunction with the presence of the electronic key it is generally preferred to require a positive act, such as pulling on the door handle, to actuate the system. A further consideration in designing such a system is the battery life for the remote unit. Since a miniature electronic key is required the battery must be small, and the unit must then be very conservative of energy to operate for an extended period. In that context, a battery life of many years is desired even though the unit is used for the control of several base stations.